Add product image gallery, harden Klaro border reset, remove attribution link

ProductGallery.tsx (main image + thumbnail strip) wired into
TodoKartenHero.tsx, only replacing the static hero photo once a
product actually has extra gallery photos — no visual change
otherwise. Backend field: Products.gallery (see payload repo).

Klaro's hard black border was still showing after the previous
color-only pass — the targeted border:none rule wasn't enough, so
this blanket-resets border/outline on every .klaro descendant and
re-adds only the shadow this theme actually wants. Also disables the
"Realisiert mit Klaro!" attribution link (BSD-3-Clause has no
on-page-attribution requirement, Klaro's own disablePoweredBy flag
covers this cleanly).

Updates the frontend README with the gallery + Klaro fixes.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/payload.ts

@@ -185,6 +185,11 @@ export type Product = {
   price: number;
   compareAtPrice: number | null;
   image: string;
+  // Extra photos beyond `image` above, for ProductGallery.tsx's
+  // thumbnail-strip UI — empty for the vast majority of products (opt-in
+  // per product, see Products.ts's own field comment). Never includes
+  // `image` itself; ProductGallery treats `image` as always-slide-zero.
+  gallery: string[];
   href: string | null;
   // `active` is opt-in for callers to filter by, not applied inside
   // getProducts()/getProductBySlug() themselves — cart, checkout, order
@@ -250,6 +255,7 @@ type PayloadProduct = {
   price: number;
   compareAtPrice: number | null;
   image: { url: string } | number | null;
+  gallery: ({ url: string } | number)[] | null;
   detailHref: string | null;
   active: boolean;
   updatedAt: string;
@@ -314,6 +320,9 @@ export function mapPayloadProduct(product: PayloadProduct): Product {
     price: product.price,
     compareAtPrice: product.compareAtPrice ?? null,
     image: typeof product.image === "object" && product.image ? product.image.url : "",
+    gallery: (product.gallery ?? [])
+      .map((g) => (typeof g === "object" && g ? g.url : null))
+      .filter((url): url is string => Boolean(url)),
     href: product.detailHref || null,
     active: product.active,
     updatedAt: product.updatedAt,